1. Field of the Invention
This invention relates to a chlorine-containing resin composition which is non-toxic or has low toxicity and which has excellent thermal stability for prolonged periods of time.
2. Description of the Prior Art
It is well known that chlorine-containing resins such as polyvinyl chloride, polyvinylidene chloride, chlorinated polyvinyl chloride, chlorinated polyethylene, vinyl chloride-vinylidene chloride copolymers, vinyl chloride-vinyl acetate copolymers, vinyl chloride-ethylene copolymers and the like tend to undergo thermal decomposition which gives rise to a coloration of the resins upon processing and, therefore, it is desirable to incorporate thermal stabilizers into such chlorine-containing resins. The main thermal stabilizers for stabilizing chlorine-containing resins are calcium compounds, which have recently come into wide-spread use as a non-toxic or of low toxicity stabilizer in place of the conventionally used lead compounds such as lead salts and lead soaps. However, stabilizers for chlorine-containing resins which satisfy the requirements that they be both effective for preventing an initial stage coloration of the resins and also be effective for achieving thermal stability over prolonged periods of time, have not yet been developed. Calcium hydroxide which is a typical example of a hydroxide has a high ability to scavenge free hydrogen chloride which is evolved during the thermal decomposition of chlorine-containing resins, but hydrogen chloride is also removed from the resin molecules upon molding due to the high basicity of calcium hydroxide, thereby accelerating deterioration and coloration of the resins. Further, calcium carbonate which is also useful as a stabilizer has only a weak ability to scavenge hydrogen chloride and free hydrogen chloride which is evolved during the thermal decomposition of chlorine-containing resins is not sufficiently scavenged, thereby accelerating the decomposition and coloration of chlorine-containing resins.
Recently, chlorine containing resin compositions having an improved thermal stability comprising, as stabilizers, calcium compounds such as calcium silicates, calcium silicates having coated thereon an organic silicon compound; calcium titanates; calcium aluminates; and the like and, optionally, organic acid salts and/or organic tin compounds have been proposed, as described in U.S. Patent Application Ser. No. 636,199, filed Nov. 28, 1975 now U.S. Pat. No. 4,052,350. These resin compsitions generally have excellent thermal stability, i.e., minimized decomposition and coloration of the chlorine-containing resin upon heating during processing of the resin compositions, but these resin compositions, particularly those containing a large proportion of such inorganic stabilizers, tend to show slightly inferior processing capability and dyeability with pigments, in particular, in dyeing the resin compositions brown and dark gray colors.